In mobile communication terminals, an antenna circuit transmits or receives a certain radio wave signal through an antenna.
Impedance is required to be accurately matched such that the antenna has optimal transmission and reception radiation performance.
Therefore, the antenna circuit includes a capacitor and an inductor. The antenna circuit controls the value of the capacitor and the value of the inductor to match the impedances of the antenna in an optimal state.
In matching impedances, the impedances of an antenna are matched in a state where a mobile communication terminal is located in a free space.
Generally, a user uses a mobile communication terminal in a state where the user has held a body with its hand and brought a speaker into contact with its ear, or uses the mobile communication terminal by using an earphone in a state where the body of the communication terminal has been placed in a bag or a pocket.
Since a user uses a mobile communication terminal in a state where the user has held a body with its hand and brought a speaker into contact with its ear or uses the mobile communication terminal in a state where the body of the communication terminal has been placed in a bag or a pocket, the impedance matching factor of an antenna varies, and consequently, the transmission and reception radiation performance of the antenna with impedances matched in a free space is degraded.
Accordingly, mobile communication terminals include an adaptive tuning antenna circuit that allows an antenna to have optimal transmission and reception radiation performance by automatically controlling the impedances of the antenna when the impedance matching factor of the antenna is changed.
To optimally maintain the transmission and reception radiation performance of the antenna, the adaptive tuning antenna circuit is required to detect the changed impedance state of the antenna.
For this, the adaptive tuning antenna circuit includes a coupler, and detects a forward power and a reflected power outputted from the coupler.
In order to detect the reflected power and the forward power and control an impedance, however, the adaptive tuning antenna circuit is required to include the coupler and a plurality of power detectors for detecting the forward power and the reflected power outputted from the coupler.